Wash kangkong thoroughly. Trim tougher stems and separate stems from leaves.

Heat oil in a skillet or wok over medium heat. Add garlic and onion and sauté until fragrant and softened, about 2–3 minutes.

Pour in soy sauce, vinegar, black pepper, and sugar (if using). Bring to a gentle simmer.

Add kangkong stems first and cook 1–2 minutes. Add leaves and cook until just wilted, about 2–3 minutes more.

Taste and adjust salt if needed. Remove from heat and serve immediately with rice.